The invention proceeds from a hydraulic displacement machine, in particular from a displacement pump, which has two components slidably movable relative to one another.
A displacement of this type, designed as an internal gear pump, is shown, for example, in DE 43 22 240 C2. In this known internal gear pump, the pinion and ring wheel enclose a crescent-shaped pump chamber, in which is located an approximately semicrescent-shaped filling piece, by means of which the high-pressure region and the low-pressure region of the pump are sealed off relative to one another along the tooth tips of the two gearwheels. For efficient sealing off, even in the event of pronounced pressure differences between the high-pressure region and the low-pressure region, the filling piece is divided longitudinally. The gap between the two filling piece parts is subjected to pressure in such a way that the two filling piece parts are in each case pressed with a slight excess of force against the tooth tips of the gearwheels.
The high-pressure region and low-pressure region of a gear machine must also be sealed off relative to one another on the end faces of the gearwheels. If the gear machine is also to be used at higher pressures and is to seal off with high efficiency, components are also used for sealing off on the end faces of the gearwheels, said components being pressed with some excess of force against the gearwheels. For this purpose, a pressure field is connected to the high-pressure region of the gear machine on the rear side, facing away from the gearwheels, of the components, which are usually designated as axial sealing disks.
The materials hitherto used for the components pressed against the gearwheels for sealing-off purposes undergo abrasive wear, particularly at high rotational speeds of the internal gear machine and when the working medium is at high pressure and at high temperatures. To be precise, the excess of force with which the components are pressed against the gearwheels is obtained essentially by means of surfaces of different size, on which the pressure acts, and therefore increases with a rising pressure. High rotational speeds and high temperatures may lead to faulty lubrication between the components and the gearwheels. The abrasion enters the hydraulic circuit and may cause damage and malfunctions.
It is possible, in principle, to remove the abrasion from the hydraulic medium by the installation of a filter. Systems where so-called stationary hydraulics operate are equipped, so to speak, as standard, with a filter. There are, however, also applications, particularly in the automotive sector, where the use of filters is to be avoided. Filters of this type gradually become clogged, consequently increase the pressure losses in the hydraulic circuit and have to be exchanged. A part is played, last but not least, by the space which would be necessary for a filter and access to it and by the additional costs of manufacturing automobiles.
Moreover, wear on the components sliding against one another cannot always be compensated by a type of adjustment, so that the internal leakages in the machine increase and efficiency losses increase.
Problems with the wear of components sliding against one another in a displacement machine arise, irrespective of specific operating parameters, such as high rotational speed or high temperature, even when the operating medium has per se poor lubricating properties. Operating media of this type are, for example, fuels, such as gasoline or diesel for internal combustion engines. Piston pumps, in particular radial piston pumps, are predominantly used for the high-pressure feed of fuels of this type.
A displacement machine of the generic type, designed as a radial piston pump and provided for the high-pressure feed of fuel, is known, for example, from DE 42 13 798 A1. In such a radial piston pump, on the one hand, the piston and cylinder, as displacement parts, slide against one another. On the other hand, one of the two displacement parts or a sliding shoe held on it slides on an eccentric ring, by means of the which the movement of the one displacement part is brought about in the feed stroke.
The object on which the invention is based is, therefore, to develop further a hydraulic displacement machine, which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in such a way that the wear on the components sliding against one another is low. In particular, when the gear pump is used in an automobile, here particularly in the region of the gear, wear-induced particles are to be discharged into the hydraulic medium only to a very slight extent and the installation of a filter or at least the exchange of a filter is to be capable of being dispensed with. When a piston pump is used for feeding fuel, the wear on the components sliding against one another is to be low, despite the poor lubricating capacity of the operating medium, so that abrasion particles do not block the injection nozzles or make them sluggish and so that a failure of the pump due to the seizure of the displacement parts or due to excessive wear on the lifting element is avoided.
In a displacement machine of the aforementioned type object is achieved, according to the invention, in that at least one of the two components is hardened at least on the surface and consists of sintered material which contains predominantly ferrite and, in addition, a constituent for improving the sliding properties. The mixing of hardenable ferrite for component strength and wear resistance with a constituent for improving the sliding properties gives rise, after sintering, hardening and a grinding process, by means of which the component acquires its exact dimensions and a smooth surface, to a component which tolerates even faulty lubrication during operation without any appreciable abrasion. As a result, the wear on the displacement machine and the discharge of particles by the latter are very low.
Pursuant to one specific embodiment of the present invention, in an internal gear machine, preferably one component is produced from sintered material which serves for sealing off a high-pressure region from a low-pressure region along the tooth tips or along the end faces of the gearwheels.
In a hydraulic piston machine, it is beneficial if, at least one of the two displacement parts of a displacement unit, specifically piston and cylinder, is produced from the sintered material hardened at least on the surface. Advantageously, at least part of the displacement part/lifting element pair is also produced from the sintered material. In this case, it should be pointed out expressly that one displacement part or the lifting element may also be of multipart design, and only one of these parts, specifically that part sliding on the counterpiece, consists of sintered material.
Preferably, the component consisting of the sintered material is hardened by nitriding, an edge zone of the component being enriched with nitrogen at temperatures of around 500 degrees Celsius, by the component being exposed to a nitrogen-discharging medium, for example a gas stream.
Nitriding per se is a generally known method for the surface-hardening of components, so that there is no need to discuss it in any more detail here.
The component contains as constituents improving the sliding properties, preferably copper, molybdenum disulfide and graphite. The requirements are satisfied particularly effectively by a combination of these constituents with one another in the proportions specified as preferred.